Embodiments of the invention relate generally to systems and methods of nuclear magnetic resonance (NMR) imaging, more particularly, to a system and method for separating the NMR signal contributions from a plurality of different species having different chemical shifts.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
Magnetic resonance imaging (MRI) is a medical imaging modality that offers remarkable image contrast between soft tissues such as fat and muscle. While this soft tissue contrast is typically the hallmark of MRI, the substantially bright signal attributed to fat often causes difficulties when imaging regions of the body that may be obscured by fat containing tissues. This can impair clinical diagnoses, however, so techniques for separating the MR signal from water and fat and suppressing the fat were developed.
A majority of techniques employed for water-fat decomposition and suppressing the fat have been developed to assume a relatively simple signal representation that models both water and fat as a single resonant frequency at approximately 3.5 ppm (210 Hz at a field strength of 1.5 Tesla and 420 Hz at a magnetic field strength of 3.0 Tesla) away from the water resonant frequency. Exemplary methods of conventional fat suppression include inversion recovery, spectral saturation (“FatSat”), and chemical-shift based multipoint Dixon methods. However, each existing/conventional method of fat suppression has limitations and drawbacks associated therewith. With respect to inversion recovery methods, such as short-tau inversion recovery (STIR), such methods suffer from reduced SNR and have mixed contrast with T1-dependence. With respect to spectral saturation techniques, such as a Chemical Shift Selective Imaging Sequence (CHESS) or “FatSat”, such methods provide non-uniform suppression and fail in the areas of high B0 inhomogeneities, particularly in large field-of-view and off-isocenter imaging. With respect to chemical-shift based multipoint Dixon methods, a variety of methods have been developed, including 2-point, 3-point, and other multi-point Dixon methods. One variant of a multi-point Dixon method is IDEAL, as described in U.S. Pat. No. 6,856,134, in which pulse sequences are employed to acquire multiple images at different echo times (TE), and an iterative linear least squares approach is used to estimate the separate water and fat signal components. However, the IDEAL method, like all other Dixon methods, models the fat signal as having one resonant frequency. While water is well modeled by a single resonant frequency, it is well known that fat has a number of spectral peaks. Thus, when fat is modeled by a single frequency, only the main fat peak will be resolved, while the remaining peaks will manifest partly as a baseline level of signal within adipose tissue, appearing as “grey fat” on the separated water images.
Recently, a multi-frequency fat spectrum model was developed for imaging spin species such as fat and water, as described in U.S. Pat. App. Pub. No. 2009/0261823. The multi-frequency fat spectrum model employs a 3-point chemical-shift based technique to completely suppress the fat signal from the water images. While such a technique acts to completely suppress the fat, so as to eliminate the “grey fat” on the separated water images, it is recognized that such a technique does have its limitations. Specifically, as the multi-frequency fat spectrum model employs a 3-point chemical-shift based technique, at least 3 separate echoes for water-fat separation need to be acquired, thereby necessitating a longer acquisition time. That is, the need to acquire at least 3 separate echoes for water-fat separation is limiting and is seen as being undesirable in that it leads to an increased pulse sequence and scan time as compared to a 2-point technique, for example.
Therefore, it would be desirable to provide a system and method for separating the NMR signal contributions from a plurality of different species having different chemical shifts, such as water and fat. It would also be desirable for the system and method to provide improved fat suppression in the water images so as to eliminate “grey fat” and increase contrast in the water images. It would further be desirable for the system and method to provide such fat suppression in an automated and time efficient manner.